Ammonia synthesis process

ABSTRACT

1. IN A PROCESS FOR SYNTHESIZING AMMONIA AT HIGH PRESSURE FROM A FEED GAS STREAM CONTAINING HYDROGEN AND NITROGEN, THE STEPS COMPRISING PASSING SAID FEED GAS STREAM IN HEAT EXCHANGE RELATIONSHIP WITH A PARTIALLY SYNTHESIZED GAS HAVING A TEMPERATURE FROM ABOUT 800 TO 1100*F. TO INCREASE THE TEMPERATURE OF SAID FEED GAS STREAM TO ABOUT 650-800*F. AND REDUCE THE TEMPERATURE OF SAID PARTIALLY SYNTHESIZED GAS, PASSING SAID HEAT EXCHANGED FEED GAS STREAM DIRECTLY FROM SAID HEAT EXCHANGE STEP TO A SYNTHESIS CONVERTER AND SUBJECTING THE SAME TO SYNTHESIS CONDITIONS TO PRODUCE PARTIALLY SYNTHESIZED GAS, AND THEREAFTER PASSING SAID HEAT EXCHANGED PARTIALLY SYNTHESIZED GAS DIRECTLY FROM SAID HEAT EXCHANGER TO A SYNTHESIS CONVERTER AND SUBJECTING SAID PARTIALLY SYNTHESIZED GAS TO FURTHER SYNTHESIS THEREIN.

Nov 26 174 g RIGHT ET AL 3,851,046

AMMONIA smmmsrs PROCESS ori inal Filed Feb. 8, 1971 2 Sheets-Sheet '1Nov. 26, 1974 1.. E. WRIGHT ET AL 335L046 AMMONIA smmssxs PROCESS w Qw 2Sheets-Sheet 2:

Origihal Filed Feb. 8. 1971 United States Patent 3,851,046 AMMONIASYNTHESIS PROCESS Lee E. Wright, West Covina, and Allan E. Pickford,Palos Verdes Estates, Califl, assignors to C. F. Braun 8; Company,Alhambra, Calif.

Original application Feb. 3, 1971, Ser. No. 113,333, now Patent No.3,721,532. Divided and this appplication Sept. 27, 1972, Ser. No.292,591

Int. Cl. C01c 1/02 US. Cl. 423--359 3 Claims ABSTRACT OF THE DISCLOSUREA system of apparatus and process for synthesizing ammonia aredisclosed, which includes first and second catalytic synthesisconverters with a heat exchanger interposed therebetween and operativelyconnected to the inlet and outlet of one converter and to the inlet ofthe other converter to permit a feed gas stream to be passed in heatexchange realtionship with a partially synthesized gas stream passingfrom the outlet of the first converter to the inlet of the secondconverter. The converters and heat exchanger are mounted on a supportplatform, with one converter and heat exchanger mounted for movementwith respect to the platform to accommodate dimensional changes causedby thermal expansion during operation.

This is a division of application Ser. No. 113,333, filed Feb. 8, 1971,and now US. Pat. 3,721,532.

BACKGROUND OF THE INVENTION This invention relates generally asindicated to the synthesis of ammonia and more particularly to acombination of apparatus and process in which such synthesis canadvantageously be carried out.

The production of ammonia from natural gas, such as methane, of course,is well known. Generally, natural gas, water in the form of steam, andair are combined in a series of chemical reactions to produce ammonia ofa high degree of purity. The chemical reactions involved in theconventional ammonia process are:

Reforming Reaction CH +H O CO+3H Shift Reaction CO-l-H O- CO +HOxidation Reaction 2H +O 2H O Ammonia Synthesis 3H +N 2NH The degree towhich these reactions go to completion is generally a function oftemperature, pressure and constituents present in the respectivereaction. While the reforming reaction is generally conducted at hightemperatures and moderate pressure in the presence of a large excess ofsteam, the ammonia synthesis reaction is conventionally conducted atvery high pressures and relatively low to moderate temperatures in orderto obtain efficient conversion, inasmuch as the conversion reaction isenhanced by high pressure and low temperature and is greatly retarded bytemperatures on the order of about 900950 F. This is, of course,particularly significant since the ammonia synthesis reaction isexothermic. Moreover, from a practical standpoint, the synthesis ofammonia is significantly limited not only by the requirement of areactor capable of withstanding high pressure but also by the necessityfor a minimum temperature of at least about 650-700 F. to initiate thereaction, which in turn necessitates careful and accurate control oftemperature.

As a result of such limiting factors, a relatively low percentage ofconversion is ordinarily obtained in a single pass through conventionalreactors. For the process to be economically feasible, however, theunconverted hydrogen and nitrogen must be recovered and subjected to icefurther conversion, but before re-entering the reactor, the temperaturemust be reduced from about 950-1 F. to about 650-700 F. In some priorsystems, this has been accomplished by passing the unconverted gasthrough one or more chillers and compressors and ultimately recycling tothe reactor. This has not resulted in a generally economical andacceptable system, since either an exremely large reactor or a veryrapid recycle rate is required to produce sufiicient conversion.

In another previously used system, a single reactor is used whichincludes two somewhat separate reaction zones and temperature reductionmeans positioned generally between the reaction zones. Such system haspractical disadvantages due to the size of reactor required andinadequate and poorly controllable temperature reduction resulting fromthe heat exchange tubes being positioned adjacent the catalyst bed inwhich heat, of course, is generated by the exothermic reaction andimparted to the tubes.

SUMMARY OF THE INVENTION It is a principal object of the presentinvention therefore to provide a combination of apparatus and theaccompanying process for the synthesis of ammonia in which substantiallycomplete conversion may be obtained without encountering the aforenoteddisadvantages. In one form, the invention thus comprises a pair ofcatalytic synthesis converters with a heat exchanger positioned therebetween and connected to the inlet and outlet of one converter and tothe inlet of the other converter. The feed gas stream may thus be passedin heat exchange relationship with a partially synthesized gas streamfrom the first converter. A support platform is also provided with oneof the converters fixedly mounted on the platform and the heat exchangerand other converter mounted thereon for movement with respect to theplatform to accommodate dimensional changes caused by thermal expansionduring operation.

Another object of this invention is the provision of an ammoniasynthesis system in which the apparatus is arranged on a supportplatform in a relatively compact manner to achieve efficient utilizationof available plant facilities.

An additional object is the provision of a system in wlicclia effectiveand efficient temperature control is pro- Vl e Yet another object ofthis invention is the provision of a system for synthesizing ammonia inwhich eflicient use is made of the best generated during reaction byproviding for heat exchange with the reaction product stream.

A further object of this invention is a provision of a system forsynthesizing ammonia in which means are included to compensate forthermal expansion during operation.

Other objects, features and advantages of this invention will beapparent to those skilled in the art after a reading of the followingmore detailed description.

DESCRIPTION OF PREFERRED EMBODIMENTS In the annexed drawings:

FIG. 1 is a top plan view of the ammonia synthesis system of thisinvention;

FIG. 2 is a side view of the system taken on line 2 2 of FIG. 1;

FIG. 3 is a perspective view of the apparatus with the platform omittedfor clarity of illustration;

FIG. 4 is an enlarged partial section view of one of the synthesisconverters;

FIG. 5 is an enlarged fragmentary view of the lower portion of theconverter of FIG. 4; and

FIG. 6 is an enlarged section view of one of the heat exchangers used inthe synthesis system.

Referring now to the drawings and more particularly FIGS. 1 through 3inclusive, the synthesis system includes a pair of generally parallel,upright converters designated by numerals 1 and 2. A heat exchanger 3 isinterposed between and operatively connected to each of the converters.As shown perhaps most clearly in FIG. 3, the heat exchanger is connectedto converter 1 through pipes 4 and 5. Pipe 6 extends from the lowerportion of heat exchanger 3 and connects to the lower portion ofconverter 2. Pipe 7 is also connected to the lower portion of the heatexchanger and functions as an inlet for the incoming hydrogen andnitrogen which is admitted thereto prior to introduction into coverter1.

In operation, a feed stream containing nitrogen and hydrogen isintroduced through pipe 7 into heat exchanger 3 and after heat exchangetherein is conveyed through pipe to converter 1. Within converter 1,synthesis of ammonia occurs by reaction under pressure between hydrogenand nitrogen, thereby producing a partially synthesized gas stream whichexits from converter 1 through pipe 4 and re-enters heat exchanger 3.Within the heat exchanger, the partially synthesized gas stream ispassed in heat exchange relationship with the incoming feed stream(which enters through pipe 7) and exits through pipe 6 through which itis conveyed to converter 2. Within converter 2, the gas stream undergoesfurther conversion, as will be described more completely hereinafter.

The system also includes a conventional steam generator 10 into whichthe effluent gas from converter 2 is taken through pipe 11. Within thesteam generator, the gaseous ammonia stream is passed in heat exchangerelationship with water to produce steam for use in the reformingoperation. The use of such a generator is optional insofar as thepresent system is concerned, but since it is necessary to lower thetemperature of the gaseous ammonia for ultimate condensation to a liquidand also since steam is needed for the reforming operation, it isobviously advantageous to utilize in this manner the heat which must beremoved from the gaseous ammonia.

After undergoing heat exchange in the steam generator 10, the efiluentammonia gas is taken through pipe 12 to heat exchanger 13. The gastravels upwardly through the heat exchanger in heat exchangerelationship in the customary manner with the incoming gas stream whichhas been introduced through line 15 into heat exchanger 16. The incominghydrogen and nitrogen gas stream thus passes from heat exchanger 16 intoheat exchanger 13 and into heat exchanger 3 through line 7 as previouslydescribed. On the other hand, the ammonia gas exits from heat exchanger13 through line 18 and passes downwardly through heat exchanger 16,whereby its temperature is further lowered, and exits through line 19for further processing. For purposes of the present invention, suchfurther processing is not illustrated and will not be described indetail since it is conventional in the art, but would include furtherreduction in temperature by passing through heat exchangers and chillersand eventual condensation and separation as a substantially pure ammoniaproudct, with the unconverted hydrogen and nitrogen being ultimatelymixed with fresh feed and recycled to the reactors.

The combination of apparatus is mounted on a platform which may be an18" to 3" thick concrete or steel platform, for example, and is thusprovided in a relatively compact arrangement for efficient utilizationof available facilities. As shown, the converters, heat exchangers andsteam generators are all mounted in generally upright position andextend above and beneath platform 25. One of the converters, in theembodiment illustrated converter 2 is fixedly mounted on the platform,whereas converter 1 is mounted on the platform, as, for example throughhanger rods or springs 26 so as to be capable of both lateral andvertical movement with respect to the platform to accommodatedimensional changes caused by thermal expansion during operation. Heatexchanger 3 is likewise mounted on platform 25 by spring means 27 andsteam generator 10 and heat exchangers 13 and 16 are mounted by similarspring means 28, 29 and 30 respectively, to permit movement tocompensate for expansion and contraction during operation. At least oneof the converters should be rigidly or fixedly mounted on the platformto prevent uncontrolled movement of the apparatus and to cause the otherapparatus to return to a normal position when the system is notoperating.

In FIGS. 4 and 5, the cross section of the catalytic converters is shownin greater detail. The converter there illustrated may be eitherconverter 1 or 2 and simply for the sake of illustration is designatedas converter 2. The internal components of the converter, however, areessentially the same, even though as illustrated, converter 2 ispreferably considerably taller than converter 1 to provide a greaterresidence time for the feed stream to undergo reaction, therebyachieving a greater percent of equilibrium conversion to ammonia. Theconverter includes a shell 40 with a liner 41 positioned therein andspaced from the shell to provide an annular passageway 42 therebetween.Within the liner 41 is a catalyst bed 43 which contains a conventionalammonia synthesis catalyst such as iron or iron in combination withmolybdenum or other such substance to increase catalytic activity. Anoutlet screen 44 is provided adjacent the lower portion of the converterto prevent the solid catalyst particles from exiting with the gasstream. The outlet screen normally will be stainless steel or a nickelbased alloy and of a sulficiently small mesh to prevent the catalystparticles from passing therethrough. An inlet 45 is provided adjacentthe lower end of the converter through which the hydrogen and nitrogencontaining feed stream enters and passes upwardly through passageway 42and downwardly through catalyst bed 43, and ultimately exits throughoutlet 46.

As shown more clearly in FIG. 5, the inlet 45 includes a nozzle neckwelded at 51 to shell 40. A layer of insulating material 52 such asceramic felt is provided adjacent outlet screen 44 enclosed by shroud 53of stainless steel or a nickel base alloy to prevent excessive transferof heat from the hot gases to the shell of the converter. The converter,of course, includes cover plates 55 and 56 connected by bolts 57 and 58to its top and bottom respectively.

Referring now to FIG. 6, the heat exchanger 3 is shown in greaterdetail. The design of heat exchanger 3 is illustrated and described morecompletely in the copending application of Allan Pickford, entitled HEATEXCHANGER and assigned to the assignee of this application, andconsequently will be described in somewhat abbreviated manner herein,although its inclusion in the present invention is of considerableimportance. The incoming hydrogen and nitrogen feed stream enters theheat exchanger through opening 60 and passes in and around tube bundle61 which is positioned within the heat exchanger and spaced from shell62. Although only two tubes are illustrated, the exchanger, of course,includes the usual bundle of tubes. Transverse baffies 63 extend acrossthe interior of the heat exchanger as shown to cause the fluid to travelback and forth across the tube bundle as it passes through theexchanger, thereby providing for more efficient heat exchange. The fluidexits from the heat exchanger through outlet 65 from which it isconveyed through pipe 5 to reactor 1.

The heat exchanger 3 also includes a nozzle assembly 66 attached at itslower portion which supports the tube sheet 67 in which the tube bundleis retained whereby the tube sheet is detached from the shell of theheat exchanger. The tube sheet is mounted on channels 68 which areconnected to inlet 69 and outlet 70 of the nozzle assembly throughbellows 71 and horizontally extending channel 72 respectively. Apartition plate 75 is provided which extends between the tube sheet 67and plate 76 at the lower portion of the nozzle assembly to preventfluid entering nozzle 69 from exiting directly through outlet 7 0without passing through the tube bundle.

Pipe 4 is connected to inlet nozzle 69 of heat exchanger 3, while pipe 6is connected to outlet nozzle 70. The fluid passing within the tubebundle is thus the partially synthesized gas stream from reactor 1 whichthen exits through pipe 6 to reactor 2. A cover plate 78 is alsoprovided at the bottom of the exchanger connected to shell 62 by bolts79.

In a preferred form, the process of the present invention operatesessentially as follows. The hydrogen and nitrogen gas feed stream entersthrough line to heat exchanger 16 at a temperature of approximately 50to 200 F. and under a pressure of approximately 1450 to 4500 p.s.i.g.,in which the temperature of the feed stream is increased toapproximately 150 F. to about 350 F. The gas feed then passes under apressure of from about 1445 to about 4475 p.s.i.g. to heat exchanger 13in which the temperature is again increased, to a temperature of about400 F. to about 700 F. The exiting gas will be under a pressure of fromapproximately 1440 to about 4450 p.s.i.g. and will be taken through line7 to heat exchanger 3. The temperature of the hydrogen and nitrogen feedstream will be increased within heat exchanger 3 to at least about 650to 700 R, which temperature is necessary to initiate reaction betweenhydrogen and nitrogen to produce ammonia. As mentioned previously, thetemperature of the feed stream entering converter 1 will be maintainedno greater than about 800 F., as the conversion reaction underequilibrium conditions is not favored by higher temperature. Thepressure of the incoming gas stream to reactor 1 will be from about 1410to about 4425 p.s.i.g. Within reactor 1, the temperature of the gasstream increases since the conversion reaction is exothermic. Thetemperature of the product gas which passes from reactor 1 through pipe4 to heat exchanger 3 will be on the order of about 800 to 1100 F. andat a pressure of about 1385 to about 4410 p.s.i.g. Within the heatexchanger 3, the temperature is lowered as previously described so thatthe temperature of the gas stream in line 6 prior to entering reactor 2will be on the order of from about 650 to about 800 F. The pressure willbe about 1350 to about 4385 p.s.i.g.

Within reactor 2, further reaction of the unconverted hydrogen andammonia occurs. In reactor I, normally about 30 to about 90 molepercent, based on the hydrogen in the fresh feed, of the incominghydrogen and nitrogen is converted with from about 70 to about 10 molepercent occurring in reactor 2. Since the reaction between hydrogen andnitrogen in reactor 2 is also exothermic, the temperature of the gasincreases to around 700 to about 1000 F. The pressure is from about 1320to about 4365 p.s.i.g.

In steam generator 10, the temperature of the gas is reduced asexplained previously to a temperature on the order of about 600 to about660 F. The pressure of the gas exiting from the steam generator isnormally from about 1260 to about 4340 p.s.i.g. After passing throughheat exchangers 13 and 16, the temperature of the ammonia gas has beenreduced to about 120* to about 250 F and the pressure is about 1250 toabout 4325 p.s.i.g.

It will be appreciated from the foregoing description that the presentinvention provides a system for synthesizing ammonia in which thetemperature can be properly controlled to insure ellicient reaction. Bythe same token, the present invention provides a relatively simple andcompact arrangement of apparatus and compensates for thermal movementand thus represents a significant improvement over previously knownconventional systems.

We claim:

1. In a process for synthesizing ammonia at high pressure from a feedgas stream containing hydrogen and nitrogen, the steps comprisingpassing said feed gas stream in heat exchange relationship with apartially synthesized gas having a temperature from about 800 to 1100 F.to increase the temperature of said feed gas stream to about 650-800 F.and reduce the temperature of said partially synthesized gas, passingsaid heat exchanged feed gas stream directly from said heat exchangestep to a synthesis converter and subjecting the same to synthesisconditions to produce partially synthesized gas, and thereafter passingsaid heat exchanged partially synthesized gas directly from said heatexchanger to a synthesis converter and subjecting said partiallysynthesized gas to further synthesis therein.

2. The process of claim 1 in which approximately 30 to about molepercent of said feed gas, based on the hydrogen in a fresh feed gasstream, is converted to am-- monia in a first reactor, and saidpartially synthesized gas stream is thereafter subjected to additionalsynthesis in a second reactor.

3. A process for synthesizing ammonia at high pressure from a feed gasstream containing hydrogen and nitrogen comprising passing said teed gasstream through a heat exchanger in heat exchange relationship with apartially synthesized gas stream having a temperature from about 8001100F. to increase the temperature of said feed gas stream to about 650-800F. and reduce the temperature of said partially synthesized gas,thereafter passing said feed gas stream directly from said heatexchanger to a first reactor and subjecting the same to synthesisconditions therein to convert from about 30 to about 90 mole percent,based on the hydrogen in a fresh feed gas stream, to ammonia, passingsaid partially synthesized gas directly from said first reactor to saidheat exchanger for heat exchange with additional feed gas stream tolower the temperature of said partially synthesized gas and to increasethe temperature of said feed gas stream, and subsequently passing saidheat exchanged partially synthesized gas directly from said heatexchanger to a second converter and subjecting the same to furthersynthesis in said second converter.

References Cited UNITED STATES PATENTS 1,329,397 2/1920 Greenwood423--359 3,615,200 10/1971 Konoki 423359 EARL C. THOMAS, PrimaryExaminer H. S. MILLER, Assistant Examiner

1. IN A PROCESS FOR SYNTHESIZING AMMONIA AT HIGH PRESSURE FROM A FEEDGAS STREAM CONTAINING HYDROGEN AND NITROGEN, THE STEPS COMPRISINGPASSING SAID FEED GAS STREAM IN HEAT EXCHANGE RELATIONSHIP WITH APARTIALLY SYNTHESIZED GAS HAVING A TEMPERATURE FROM ABOUT 800 TO 1100*F.TO INCREASE THE TEMPERATURE OF SAID FEED GAS STREAM TO ABOUT 650-800*F.AND REDUCE THE TEMPERATURE OF SAID PARTIALLY SYNTHESIZED GAS, PASSINGSAID HEAT EXCHANGED FEED GAS STREAM DIRECTLY FROM SAID HEAT EXCHANGESTEP TO A SYNTHESIS CONVERTER AND SUBJECTING THE SAME TO SYNTHESISCONDITIONS TO PRODUCE PARTIALLY SYNTHESIZED GAS, AND THEREAFTER PASSINGSAID HEAT EXCHANGED PARTIALLY SYNTHESIZED GAS DIRECTLY FROM SAID HEATEXCHANGER TO A SYNTHESIS CONVERTER AND SUBJECTING SAID PARTIALLYSYNTHESIZED GAS TO FURTHER SYNTHESIS THEREIN.